Over the past several years, LED lighting has delivered huge improvements in energy efficient lighting while simultaneously enabling high quality. Another feature of LED lighting is the very long life of the lighting product, typically with an L70 (the amount of time a light source will operate before its lumen output drops to 70% of its initial output) greater than 25,000 hours. By contrast, the incumbent light sources LEDs replace typically have L70 of 10,000 hours (fluorescent lighting) or catastrophic failure at about 1000 hours (incandescent). This long life is a function of all components in the LED light source, but especially the blue or violet emitting diode and the down-converting phosphor materials which absorb light emitted by the diode and convert it to other colors to complete the visible spectrum. When the diode degrades, the LED becomes less bright, but retains its color balance. However, when a phosphor degrades the LED typically becomes less bright and also loses its color balance. The LED may take on a non-white hue. Color shift is typically more problematic from the user perspective.
The most common architecture for LED packages is to disperse a phosphor material in a silicone matrix to form a slurry and deposit this slurry into a reflective cup area which also includes the light emitting diode. The package has two types of reflective areas, a diffuse reflective surface typically made of plastic or ceramic and a more specular reflective surface which is formed from the electrical contacts. Typically, the specular reflective surfaces are plated with silver to significantly enhance the reflectivity and increase light extraction from the package.
An LED phosphor is usually made up of an activator ion, typically divalent europium or trivalent cerium, in a host. The activator ion directly absorbs the incoming light and emits light of a longer wavelength in a process typically called down-conversion. That is, an incident photon is down-converted from a higher energy blue photon to a lower energy photon, such as cyan, green, yellow, orange, or red. The host helps tune the absorptive and emissive wavelengths of the activator. Additionally, the degree of crystallinity of the host around the activator can play a large role in the efficiency of absorption and emission.
Phosphor degradation is typically attributed to the action of water or oxygen in the presence of the heat and light produced by the LED. It has become common practice to coat many types of phosphors with a layer to prevent the water or oxygen from coming in contact with the phosphor and facilitating degradation. Typically, these coatings are inorganic oxides, and are deposited on the phosphor either by a solution phase, e.g. sol-gel, reaction, or a vapor phase reaction.
A phosphor can typically degrade through three mechanisms. First, oxidation of the activator can eliminate its 4f orbital to 5d orbital charge transfer absorption, rendering it unable to absorb the incident light. Second, the host may deform chemically, changing the energy of the activator's absorption and emission. Third the host may deform physically, losing crystallinity around the activator, and decreasing the efficiency of the activator's absorption and emission. Typically, a low temperature chemical change to the host will also result in a loss of crystallinity. The overall impact of these degradations is dependent upon the extent to which the host, in either its pristine or degraded state, allows the water or oxygen to permeate through the material. For example, degradation of cerium doped yttrium aluminum garnet phosphor materials is very slow relative to europium doped alkali earth orthosilicate phosphors.
Unlike oxide and nitride phosphors, which make up almost the entirety of commercially used phosphors, sulfide-based phosphors (e.g., thioaluminate phosphors) add an additional LED failure mechanism. A hydrolysis reaction with water can release sulfur from the phosphor which can degrade performance by, for example, tarnishing the reflective surfaces in the LED package. This blackening can vastly decrease the light output of the phosphor-converted LED. Thus, failure to properly coat sulfide phosphors can create a larger problem than failure to properly coat oxide or nitride phosphors.
The ability to effectively coat sulfide phosphors is hindered by the differences in surface chemistry between these phosphors and their oxide or nitride analogues.